Obtaining cross-linked polyurethanes from a two-component system in an organic solvent medium or in an aqueous medium in dispersion, starting from a polyol resin by reaction with a polyisocyanate, is already well known. More particularly, acrylic polyols are known for coating applications because of their better performances, in particular as regards ageing. Such polyols are copolymers of a blend of acrylic monomers comprising a hydroxyalkyl (meth)acrylate, such as hydroxyethyl (meth)acrylate (HE(M)A), or hydroxypropyl (meth)acrylate or hydroxybutyl (meth)acrylate.
The essential disadvantage of such prior art systems is linked to the fact that an isocyanate (polyisocyanate) is indispensable as a cross-linking agent for such cross-linked two-component polyurethane systems based on acrylic-polyol resins. In fact, such use gives rise to problems with toxicity, safety and noxiousness to human health and to the environment in general; these problems impose severe restrictions as regards their handling, even in aqueous media, and so handling becomes ever more complex and expensive. Because of their toxicity and their preparation from starting materials which are also toxic and noxious to the environment, such as phosgene with the emission of hydrochloric acid, which is also noxious to the environment, chemistry of this type, based on the use of isocyanates, needs to be replaced with solutions which are more environmentally and human-friendly and which respect the environment and human health, such as sustainable development using new starting materials which allow this.
Furthermore, in addition to health and environmental problems, using a cross-linkable polyol-isocyanate system is extremely sensitive to the conditions of application, i.e. whether a solvent or an aqueous medium (even higher isocyanate consumption) is used, with consumption of some of the isocyanate functions by the residual water in a solvent medium or by the water in an aqueous medium with a stoichiometry which is difficult to control, with consequences for the reproducibility of the final performance resulting in an over-consumption of isocyanate compared with the necessary stoichiometry. The consumption of isocyanate in the system by ambient moisture or by water in an aqueous medium with secondary reactions (decarboxylation and formation of polyamines transformed into polyureas) may affect the structure and final performances of the coating. In particular, the release of CO2 by reaction with water leads to the formation of bubbles of CO2 (defects) in the final coating, in particular in the case of thick coatings. This is a major limitation to the conventional polyol-isocyanate system in terms of the possible maximum dry thickness without said defect (CO2 bubbles) for a conventional PU coating. This maximum dry thickness performance without said defect is known as the “pinhole limit” measured by the pinhole limit test as defined in the experimental section. In coating systems based on conventional PU (isocyanate-based), this maximum thickness is 70 μm at best. In the coatings of the invention, these defects are not present, and thus there is no limit. The other disadvantage of isocyanates in these coatings is their high impact on the cost price, accentuated in particular by the over-consumption of isocyanate by the secondary reaction with water. Novel two-component systems are already known in the art that do not use a polyisocyanate (known as NIPU: non-isocyanate PU) for replacing PU systems based on polyol-polyisocyanate. They are based on resins carrying cyclic carbonate groups that can be cross-linked by a polyamine.
More particularly, BE 1009543 describes a system based on carbonate-functionalized acrylic resin obtained by solution polymerization cross-linked with a primary polyamine for clear coat applications in a solvent-based medium.
US 2012/0251730 describes a two-component solvent-based composition that is capable of drying without using polyisocyanates or without melamine-formaldehyde resins as cross-linking agents. It is constituted by a copolymer obtained by radical polymerization and containing at least one cyclic carbonate group, a polyamine containing at least two primary and secondary amine groups, optionally at least one solvent, pigment and additive. The examples show a film with an interesting hardness, but not the production of compositions with a low VOC level (VOC: volatile organic compounds).
WO 2011/035982 describes a system based on acrylic resins that can be dispersed in water with a carbonate functionality for cross-linking with polyamines, the synthesis being carried out in a solvent medium. Dispersibility in water is only obtained using a polymerizable surfactant.
US 2012/316286 describes a similar hydrodispersible binder based on a vinyl copolymer obtained from a cyclic carbonate-functionalized monomer, a monomeric emulsifying agent and another co-monomer which can be used to obtain aqueous dispersions.
Although a system as described in the documents cited from the prior art can primarily be used to obtain polyurethanes (PU) without making use of a polyisocyanate, several supplemental problems have to be overcome in addition to those cited above. In particular, in a solvent medium, an acrylic resin carrying cyclic carbonate functions has a much higher viscosity than an equivalent hydroxylated resin for the same quantity by weight of monomer carrying the corresponding function, which means that the viscosity is significantly higher for the same molar quantity (or same index, expressed in equivalent mg KOH/g). This has the result of limiting the dry extract content (or solids extract) in the cross-linkable composition, this being in order to be able to keep the viscosity constant and appropriate, which is an essential parameter for applications with coatings in a solvent medium. In a solvent-based medium, introducing carbonate functions increases the viscosity, which makes the development of resins with a high dry extract and thus a low VOC level difficult. Emulsification in the form of an aqueous dispersion is a solution to this problem.
At a time when regulations regarding reducing the VOC are becoming stricter, satisfactory solutions to this problem are not known in the art. Thus, there is a need for novel resins, in particular acrylic resins, with carbonate functionalities that are capable of forming stable aqueous dispersions and that can be used for the formulation of aqueous coatings and in particular paints, varnishes, inks, adhesives, glues, sealants with a low VOC level and with an aqueous dispersion obtained thereby which is adapted to applications in coatings in aqueous media.